A fuel cell system includes a fuel cell that generates electric power via an electrochemical reaction between a fuel gas such as a gas containing hydrogen and an oxidizing gas such as air, an oxidizing gas supply section (i.e., an air compressor or the like) for supplying the oxidizing gas to the fuel cell, and a secondary battery capable of being charged and discharged. When a fuel cell system is to be started in a low-temperature environment, power-generating performance of the fuel cell and output characteristic of the secondary battery may be degraded.
In consideration of this situation, the fuel cell system described in JP 2004-281219 A is devised to include a power distribution section that supplies electric power generated by a fuel cell to accessory devices required for power generation by the fuel cell and to a secondary battery for its charging, or that supplies electric power discharged by the secondary battery to the accessory devices; and a control section that performs warm-up control processing for warming up the fuel cell and the secondary battery by alternately performing first processing and second processing at system startup. In the first processing, the power distribution section is controlled to supply generated power of the fuel cell to the accessory devices and the secondary battery, and, in the second processing, the power distribution section is controlled to supply generated power of the fuel cell and discharged power from the secondary battery to the accessory devices.
In the fuel cell system described in JP 2004-281219 A, although it may be possible to warm up the fuel cell and the secondary battery quickly by alternately performing the first processing and the second processing, the possibility that overcharge and over-discharge may occur in the secondary battery as a result cannot be denied. For example, while the drive state of an oxidizing gas supply section composed of an air compressor or the like must be controlled in order to cause the fuel cell to generate power, at the time of transition between the first processing and the second processing, there are possibilities that a change in a target value of the rotational frequency of the oxidizing gas supply section cannot be accurately followed by a change in the actual rotational frequency. The reason for this is that a time lag is generated in a change in the actual rotational frequency of the oxidizing gas supply section with respect to a command signal from the control section regarding a change of rotational frequency of the oxidizing gas supply section. In other words, the change is effected with an error. A change of rotational frequency of the oxidizing gas supply section corresponds to a change in the power generation level of the fuel cell. Accordingly, when the power generation level of the fuel cell is lowered, an undershoot may occur, resulting in an excessive decrease in the actual power generation level with respect to the target power generation value. On the other hand, when the power generation level of the fuel cell is increased, an overshoot may occur, resulting in an excessive increase in the actual power generation level with respect to the target power generation value.
When an undershoot of the power generation level of the fuel cell occurs, during an increase in power discharge from the secondary battery for compensating the decrease in power generation of the fuel cell, an over-discharge exceeding an upper threshold of power discharge tends to occur. On the other hand, when an overshoot of the power generation level of the fuel cell occurs, during an increase in power charge of the secondary battery which is charged by receiving a supply of generated power, an overcharge exceeding an upper threshold of power charge tends to occur. Such occurrences of over-discharge and overcharge of the secondary battery cause early deterioration of the secondary battery, and are therefore undesirable. Over-discharge and overcharge tend to occur particularly when the secondary battery is discharged to a level close to an upper threshold of power discharge and when the secondary battery is charged to a level close to an upper threshold of power charge.
Under a low-temperature environment, performance of the secondary battery may be degraded not only at the time of system start-up but also during continuous power generation by the fuel cell. Accordingly, in such a case, it is desirable to similarly perform the warm-up control processing to warm up the secondary battery.
An object of the present invention is to reduce possibilities of occurrence of overcharge and over-discharge of a secondary battery in a fuel cell system when the secondary battery is warmed up by alternately performing the operation of supplying generated power from a fuel cell to the secondary battery and the operation of discharging the secondary battery.